Seriously About Pronation and Kick Serve #1

Anatoly Antipin2. The Tennis Serve The serve should be one of the easiest shot. It can be done from standard position. There are no hurry, no running involved etc. On the other hand, the serve is one of the most difficult shot in tennis to learn, because there is still no clear explanation on how to build the proper serving routine. Further, I’ll try to explain the most important and difficult to understand elements of the tennis serve. Andy Roddick possesses one of the best serves in the world. There are some data of his performance. According to Figure 2.1 the Roddick’s right arm generates 80% of the ball speed. All others limbs actually are not very important and contribute merely 20%. Very famous American coach Vic Braden stated, "I still hear some television announcers telling the viewing audience that the server is getting such great power on the serve because he/she is jumping up to the ball and getting full extension. But, one of our coaches, John Tichy, served a 124 mph serve while on his knees. The ball went into the service box as it had enough topspin to force the proper trajectory". That’s why I mostly pay attention on the right arm and its parts actions.

Figure2.1. Body parts contributions to the Andy Roddick serve

To go any further, we need to know a little bit about biomechanical terminology (Figure 2.2)

Figure 2.2. Biomechanical terms of the arm movement

In the tennis slang, the pronation means the counterclockwise rotation of the arm (not just forearm) and the supination is the arm clockwise rotation. The forearm pronation has restricted range around 180°. If your palm is facing the floor you basically cannot pronate at all. To make the pronation possible you should supinate first. If the palm is facing the ceiling, you can produce the most efficient pronation with range around 180°. The upper arm also can pronate/supinate around 180°. This motion also is called: the internal/external rotation of the shoulder. The forearm and upper arm together can provide pronation/supination around 360°. In case of the any tennis serve, pros usually supinate first to provide appropriate pronation (around 90°). It is obvious; to maximize the pronation angular speed we should use both: forearm and upper arm counterclockwise rotation.

Definition: The Target Plane is the plane, which includes the tennis ball during impact and the imaginary target inside of the deuce or ad tennis court. This plane should be parallel to the perpendicular to the racquet string bed during the impact. The Target Plane basically determines the boll velocity direction. We shouldn’t change amount of the pronation, because it is almost impossible to control, in order to change boll’s direction, much easier to alter direction of the Target Plane. Always keep the range of the arm pronation around 90°.

2.1 The Basic Kick Serve Routine and PronationAlmost all of the modern instructions advise the tennis player to drop the racket in a backscratch position, provide appropriate supination, and swing up on edge like you are trying to use the side of the tennis racket to cut the ball in half. At the last second before impact, the player has to pronate the arm around 90° very quickly. In most cases the Continental grip is recommended. There are also a lot of words about legs, shoulders, trunk positioning and motion, which I’m not going to scrutinize here in detail. The pictures (Figure. 2.3) show the set of the video’s frames (last second before impact) taken during the pro Florent Serra’s typical kick serve and practically confirm instructions above.Let’s analyze these pictures and try to figure out what is really vital for the typical kick serve.Figure 2.3. Set of the pictures around impact Florent Serra’s kick serveWhat can I state about the body rotation? It looks like the body is more or less frozen (because it is very slow) and hence, it cannot contribute anything significant to the racquet’s velocity. But the arm itself and its parts are rotating in the different planes with the visible angular speeds. The arm is rotating in the vertical plane (Figure 2.3) by using mostly shoulder joint (also very slow joint). This vertical plane should be parallel to the Target Plane to provide appropriate direction of the ball’s velocity. There is also the wrist ulnar deviation, which directs the racquet upward, but this movement is not very important for the ball speed because it creates just brushing (spin) motion and I describe it later (see step 2.2). On the picture 2.3.1 the vertical ray with arrow indicates starting point of the arm vertical rotation. All others pictures include this starting point ray and its own ray for measuring angular movement of the arm between starting point and current position of the arm (the angle ϴ). The numbers next to these rays show degree of the angle ϴ as result of the vertical arm rotation. On pictures from 2.3.3 to 2.3.7 the symbol ΩV represents angular speed of the arm for particular frame, ft is time elapsed between any two consecutive frames, ft=3.33 msec.
During this vertical rotation from Figure 2.3.1 to Figure 2.3.7 the arm travels 11° (Figure 2.3.7, ϴ =11°). The arm vertical rotation angular speed ΩV practically is constant on all pictures. It varies from 1.5°/ft to 2°/ft. The shoulder joint muscles do not produce any arm acceleration, it moves like a car coasts in neutral. For the reason that the arm is moving with constant speed, the acceleration was achieved on previous steps of the serve, mostly, thanks to the fast elbow extension. The previous forearm movement forced the arm to move parallel to the Target Plane with angular speed approximately ΩV=2°/ft. At the same time, the arm pronation moves the racquet in the horizontal plane around 90°. Usually pros pronate something from 80° to 110°. Suppose the pronation provides 110° path of the racquet. It means the racket rotates in horizontal plane 10 times as many as the arm and racquet moves in vertical plane (ϴ=11°). The average horizontal angular speed will be around ΩH=20°/ft, or 10 times as many as the vertical angular speed ΩV =2°/ft. Wow, this result is astonishing!Question: Have legs, shoulders, and trunk motions contributed anything to the racquet horizontal rotation (pronation)? Answer: These parts of the body produce something to the arm and the racquet vertical rotation, but they are arguably even counterproductive for the horizontal rotation since the trunk rotates (clockwise), in opposite direction to the arm pronation (counterclockwise). OK, it looks like I found the winner! The pronation can provide much bigger angular speed than others body limbs (except the wrist) altogether.Not so fast. In reality, we are interested in the linear velocity (the speed and direction) of the racquet, not just in the angular speed.Definition: Linear speed = radius × angular speed. The direction of this velocity is perpendicular to the radius of the rotation in the plane where the point of contact rotates.
The angular speed already discussed above. But, what is the radius? The figures 2.4; 2.7 give an idea about calculation of these radiuses.

I'm slightly missing the overall point of this interesting article, too?

However, LeeD, you've got to admit that some posters in the past claimed that "pronation" doesn't happen on kick serves. Watching slow motion video of Roddick hitting 2nd serves, it was apparent to me that it was still happening prior and through contact.

Definition: Racquet efficient length Relis the distance between player’s hand (point Oon the Figures (2.4-2.7) and the ball during impact. I think Rel = 25” (63.5 cm) in the most occasions.Definition: Arm efficient length Ael is the distance between shoulder joint and player’s hand. Since everybody have different arm size, I guess Ael = 25” (63.5 cm) as average length.On the pictures above,RVis the radius of the arm and the racquet vertical rotation,RH – the radius of the racquet horizontal rotation (pronation). RV = Ael + Rel × cosβ =25” × (1+ cosβ), where:Notation: pronation angle β (very important) is the angle between long axis of the racquet and axis of the forearm/arm (Figure 2.4-2.7). RH= Rel× sinβ = 25” × sinβRV can vary fromAel to Ael + Rel (or from 25” to 50”) because cosβ has range from 0 to 1, depending on the β magnitude. RVcan never be equal to zero, because Ael or the arm efficient length is constant and equal to 25”. RHcan vary from 0 to Rel(or from 0” to 25”) becausesinβ has range from 0 to 1.RHcan be equal to zero and therefore linear speed would be zero! It can be very big problem for the tennis player.Maintaining the proper magnitude of the angle β before impact is absolutely crucial for pronation! On figures from 2.4 to 2.7 the best players keep β from 35° to 45°depending on the serve type.
How they are able to do that I described in step 2.2.2. Notation:|VLV| - Linear speed of the racquet in the vertical plane;|VLH| - Linear speed of the racquet in the horizontal plane.VLV and VLHare corresponding velocities. Reminder: thelinear speed = radius × angularspeed. In the last formula the angular speed should be expressed in radians. The angular speeds in degrees (from Figure 2.3) were:ΩV=2°/ft,ΩH=20°/ft. In radians they are ΩV=(π/90)/ft, ΩH= (π/9)/ft.Then linear speeds in the vertical and horizontal planes can be calculated according to the following formulas:|VLV|= RV× ΩV= 25” × (1+ cosβ) × (π/90)/ft = 25” × (1+ cosβ) × (π/90) × 300/sec|VLH|= RH× ΩH= 25” × sinβ × (π/9)/ft = 25” × sinβ × (π/9) × 300/sec The sum of the linear racquet speeds would be|VLV|+ |VLH|. The results of the calculation are presented on the Figure 2.8Figure 2.8. Linear speeds of the racquet in vertical |VLV|and horizontal |VLH| rotations and their summation

The data on Figure 2.8 demonstrate, if the angle β ≥ 12°the linear speed of the pronation |VLH|begins to prevail over the linear speed of the vertical rotation |VLV|.It should be noted, unfortunately, the calculated above pronation linear velocity determines mostly theoretical potential maximum. In reality, this speed may be slower even in case when the pronation angle beta has appropriate value. I’ll explain this phenomenon later, in the step 2.2.3.Since,RH = 25” × sinβ, then we can calculate the pronation efficiency according to following formula Pronation Efficiency = sinβ×100%.
The results of the calculation are presented on the Figure 2.9.Figure 2.9 Pronation’s efficiency as function of the angle β

OK, it appears I found the proof!In case of the kick serve, the pronation can really provide much bigger linear speed of the racquet than others body limbs (except the wrist) altogether! But, if the pronation angleβ=0°, the pronation produces nothing at all, just the proper racquet string bed orientation. That’s why I repeat again, the best tennis players keep the pronation angle β around 30° - 45° (Figure 2.4 -2.7). Maintaining the proper magnitude of the angle β before impact is absolutely crucial for pronation! If the pronation angle has the proper magnitude the pronation would be the most important and effective contributor to the powerful kick serves! To be continued, see please post #19.

It took three shots of tequila to make my brain stop hurting after reading that. In fact I am already too drunk to type on my phone so I am having....whats your name? (Kelly).....I am having Kelly type for me. I don't know her, I think she is related to my hot gf. Where is she anyway...mumbles and stumbles away leaving me with his phone. The end.

You watched ONE player known for his big fast flat serves, and you decide ALL servers need pronation for a kick serve? Did I get that right?

Click to expand...

Maybe you don't watch much tennis these days, but Roddick is known for more than his 1st serve. His second serve is known for both speed and SPIN.

The videos I watched were of Roddick's 2nd serve. His topspin, topspin slice, and kick serves. Study by John Yandell showed that Roddick was not only surpassing the speed of other players but matching or exceeding the spin on the ball.

So, yes, I'm guilty of basing my judgment on slow motion second serves of one of the greatest servers in history. Later, looking at 2nd serves of Sampras, I saw the same thing. So, I'll stand by my judgment that if two of greatest servers in history (and greatest 2nd serve servers) are pronating into the shot, it is something I should try to emulate.

Maybe you don't watch much tennis these days, but Roddick is known for more than his 1st serve. His second serve is known for both speed and SPIN.

The videos I watched were of Roddick's 2nd serve. His topspin, topspin slice, and kick serves. Study by John Yandell showed that Roddick was not only surpassing the speed of other players but matching or exceeding the spin on the ball.

So, yes, I'm guilty of basing my judgment on slow motion second serves of one of the greatest servers in history. Later, looking at 2nd serves of Sampras, I saw the same thing. So, I'll stand by my judgment that if two of greatest servers in history (and greatest 2nd serve servers) are pronating into the shot, it is something I should try to emulate.

Click to expand...

The pronation always has to be used, at least, to provide the proper racquet string bed orientation. It should be perpendicular to the Target Plane. The best servers also use it to create pronation flat component of the racquet speed. There is no serve without pronation in professional tennis! In case of the spin serve we can use the wrist ulnar deviation. To increase the flat component of the racquet speed we can also use the wrist flexion. The best servers utilize all of these motions.

Thank you for posting this. However, I just don't understand it...any chance you can dump it down for the non-math oriented folk? Maybe..."here are the 3 things you need to do to hit a great kick serve"

Thank you for posting this. However, I just don't understand it...any chance you can dump it down for the non-math oriented folk? Maybe..."here are the 3 things you need to do to hit a great kick serve"

Definition: Racquet efficient length Relis the distance between player’s hand (point Oon the Figures (2.4-2.7) and the ball during impact. I think Rel = 25” (63.5 cm) in the most occasions.Definition: Arm efficient length Ael is the distance between shoulder joint and player’s hand. Since everybody have different arm size, I guess Ael = 25” (63.5 cm) as average length.On the pictures above,RVis the radius of the arm and the racquet vertical rotation,RH – the radius of the racquet horizontal rotation (pronation). RV = Ael + Rel × cosβ =25” × (1+ cosβ), where:Notation: pronation angle β is the angle between long axis of the racquet and axis of the forearm/arm (Figure 2.4-2.7). RH= Rel× sinβ = 25” × sinβRV can vary fromAel to Ael + Rel (or from 25” to 50”) because cosβ has range from 0 to 1, depending on the β magnitude. RVcan never be equal to zero, because Ael or the arm efficient length is constant and equal to 25”. RHcan vary from 0 to Rel(or from 0” to 25”) becausesinβ has range from 0 to 1.RHcan be equal to zero and therefore linear speed would be zero! It can be very big problem for the tennis player.Maintaining the proper magnitude of the angle β before impact is absolutely crucial for pronation! On figures from 2.4 to 2.7 the best players keep β from 35° to 45°depending on the serve type.
How they are able to do that I described in step 2.2.2. Notation:|VLV| - Linear speed of the racquet in the vertical plane;|VLH| - Linear speed of the racquet in the horizontal plane.VLV and VLHare corresponding velocities. Reminder: thelinear speed = radius × angularspeed. In the last formula the angular speed should be expressed in radians. The angular speeds in degrees (from Figure 2.3) were:ΩV=2°/ft,ΩH=20°/ft. In radians they are ΩV=(π/90)/ft, ΩH= (π/9)/ft.Then linear speeds in the vertical and horizontal planes can be calculated according to the followin g formulas:|VLV|= RV× ΩV= 25” × (1+ cosβ) × (π/90)/ft = 25” × (1+ cosβ) × (π/90) × 300/sec|VLH|= RH× ΩH= 25” × sinβ × (π/9)/ft = 25” × sinβ × (π/9) × 300/sec The sum of the linear racquet speeds would be|VLV|+ |VLH|. The results of the calculation are presented on the Figure 2.8Figure 2.8. Linear speeds of the racquet in vertical |VLV|and horizontal |VLH| rotations and their summation

The data on Figure 2.8 demonstrate, if the angleβ ≥ 12°the linear speed of the pronation |VLH|begins to prevail over the linear speed of the vertical rotation |VLV|.It should be noted, unfortunately, the calculated above pronation linear velocity determines mostly theoretical potential maximum. In reality, this speed may be slower even in case when the pronation angle beta has appropriate value. I’ll explain this phenomenon later, in the step 2.2.3.Since,RH = 25” × sinβ, then we can calculate the pronation efficiency according to following formula Pronation Efficiency = sinβ×100%.
The results of the calculation are presented on the Figure 2.9.Figure 2.9 Pronation’s efficiency as function of the angle β

OK, it appears I found the proof!In case of the kick serve, the pronation can really provide much bigger linear speed of the racquet than others body limbs (except the wrist) altogether! But, if the pronation angleβ=0°, the pronation produces nothing at all, just the proper racquet string bed orientation. That’s why I repeat again, the best tennis players keep the pronation angle β around 30° - 45° (Figure 2.4 -2.7). Maintaining the proper magnitude of the angle β before impact is absolutely crucial for pronation! If the pronation angle has the proper magnitude the pronation would be the most important and effective contributor to the powerful kick serves!

Click to expand...

Toly, this is an excellent and very thoughtful analysis. Isn't there one additional degree of freedom though, which is extension of the arm (that is, a rapid change in the beta angle as the arm is straightened)? The serve is really the most complex stroke in tennis.

There are a lot of speculations about the wrist movement (the wrist snap, the wrist whip effect) during the last second before impact. Some of the tennis specialists (Vic Braden etc) say no such thing occurs. But, others (Brian Gordon and so on) insist the wrist motion is very important. But what do the arm and the wrist in particularly really do? When I’m serving I feel like my wrist is doing something very essential, however maybe my feelings mislead me. But, the pictures never lie.

This time, I’m going to analyze the pictures from Figure2.10 and Figure 2.11. They show the set of the video’s frames taken during the Lleyton Hewitt and Andy Roddick kick serves. Let’s pay attention to the wrist motion

Figure 2.10. The wrist at the last second before impact Lleyton Hewitt’s kick serveFigure 2.12. The wrist at the last second before impact Andy Roddick’s kick serve

These players employ the Continental grip. From above pictures we can see the racquet string plane has practically constant vertical orientation. On all pictures, the hand is almost straight out and there is no extension or flexion of the wrist before and for the duration of the impact. At the same time the wrist ulnar deviation directs the racquet upward very fast. The pictures show this movement takes place in the plane which coincides with the racquet string plane. Hence, it can produce the brushing boll motion only. But, the brushing motion mostly responsible for the ball rotation, not for the ball speed. The wrist ulnar deviation might be used to create different types of the spin serves and this is a very good option. Nevertheless, it also can decrease pronation efficiency, the most important part of the tennis kick serve.
OK, so far I don’t see any wrist actions in the kick serve which could add any real speed to the tennis ball, but only the spin!To be continued, see please post #62.

i get the pics but the whole data and the funky numbers and all that stuff is over my head. do you really need all that to learn how to hit a kick serve? i mean ****, cont or ebh grip, throw ball up. hit from 7 to 2 on the ball. and if you did everything right you should be able to tell the time if you were wearing a watch on the racket hand.
step two. practice.
thats how i learned anyways.

i get the pics but the whole data and the funky numbers and all that stuff is over my head. do you really need all that to learn how to hit a kick serve? i mean ****, cont or ebh grip, throw ball up. hit from 7 to 2 on the ball. and if you did everything right you should be able to tell the time if you were wearing a watch on the racket hand.
step two. practice.
thats how i learned anyways.

Click to expand...

The key in your statement is “… if you did everything right “. The problem is what is right and what is wrong? I can hit kick serve without pronation at all. For example, ITF Coaches Education Program, (Biomechanical principles for the serve in tennis, 2007) stated, “Pronation of forearm does not gives power but realigns the racket face”. See link http://www.itftennis.com/shared/medialibrary/pdf/original/IO_24976_original.PDFIt means the arm pronation is not important. There is no correlation between my analysis and official professional data. My calculation approach to the real data (videos) convinced me that pronation is the most important part of the kick serve. Somebody should have done that. I did it myself and you are blaming me for that.

OK, it appears I found the proof!In case of the kick serve, the pronation can really provide much bigger linear speed of the racquet than others body limbs (except the wrist) altogether! But, if the pronation angleβ=0°, the pronation produces nothing at all, just the proper racquet string bed orientation. That’s why I repeat again, the best tennis players keep the pronation angle β around 30° - 45° (Figure 2.4 -2.7). Maintaining the proper magnitude of the angle β before impact is absolutely crucial for pronation! If the pronation angle has the proper magnitude the pronation would be the most important and effective contributor to the powerful kick serves!

Click to expand...

Correct if I am wrong here: a β of 0° results in no added acceleration due to pronation, but in maximum impact height. a β of 90° results max racket head speed due to pronation, but this speed is contributed only in hitting across the ball and in minimal impact height. 30-45° combines a compromise: impact height a bit lowered, but good racket head acceleration from pronation, going up and across the ball, which is needed for the kick serve.

The key in your statement is “… if you did everything right “. The problem is what is right and what is wrong? I can hit kick serve without pronation at all. For example, ITF Coaches Education Program, (Biomechanical principles for the serve in tennis, 2007) stated, “Pronation of forearm does not gives power but realigns the racket face”. See link http://www.itftennis.com/shared/medialibrary/pdf/original/IO_24976_original.PDFIt means the arm pronation is not important. There is no correlation between my analysis and official professional data. My calculation approach of the real data (videos) convinced me that pronation is the most important part of the kick serve. Somebody should have done that. I did it myself and you are blaming me for that.

Thanks toly. Great analysis. May I ask: Is this yours? If so, are you planning to write a book? Otherwise, if not, what is your source? Looks very analytical and interesting...

Click to expand...

Thanks gzhpcu. You are always very supportive. I’m very old retired Russian man. I just got nothing to do. TIW, I’m writing this stuff. My source is my head and videos from internet. I quit reading professional articles. There is mostly pure speculation or misleading information, see my previous post. About book, I don’t know.

The arm pronation can produce angular speed around Ω = 6000°/sec, trunk rotates around 500°/sec, arm rotates in vertical plane around 600°/sec, etc. What motion is the most important? There is no uncertainty, at least for me, it should be pronation and nothing else.

Correct if I am wrong here: a β of 0° results in no added acceleration due to pronation, but in maximum impact height. a β of 90° results max racket head speed due to pronation, but this speed is contributed only in hitting across the ball and in minimal impact height. 30-45° combines a compromise: impact height a bit lowered, but good racket head acceleration from pronation, going up and across the ball, which is needed for the kick serve.

If you do, you probably know that a human is much more complex than a kinematic model with rods and joints. Different people have different strength and stamina of different muscles, different flexibility, different elasticity of different tendons, different natural preferences leading to different control over various motions, leading to vastly different results.

I am not against your kinematic modeling - it is a fun exercise that can produce useful insight into what factors affect tennis strokes. But the idea that you can come up with optimal solutions by kinematic modeling is a gross oversimplification. It misleads people into thinking that there is one optimal way of doing things in tennis, which any seasoned tennis player/coach would disagree with.

This is hilarous.
I few guys who don't play tennis trying to figure out the mechanics of a kick serve. :twisted::twisted: Who are you trying to kid here.
Why take anything from anyone who CAN hit a kick serve?

But all i ever heard her say when it came to spin generation was Low to High. On a kick serve i believe that means you make your 6 to 12, or 7 to 2, type contact and work it.

Everything else boils down to practice and human instinct.

I tried pronation once on a kick, the ball flew over my partners head and went out so wide it had everybody in the club scared.

Has anyone else begun to get disgusted with the over analysis of kick serves lately? What ever happened to throwing the ball behind your head and hitting up. that was always the most effective for me.

Now if only it REALLY were that simple

Click to expand...

no wonder you're kick is bad. I mean, we all need to be physics majors to play a sport where you aren't pluggin in numbers during the match, right? :roll:

the topspin serves are the ones I naturally hit, I never used these complicated formulas to hit them. It was natural (though still need a lotta improvement). this stuff is useless, garbage, when it comes to helping people understand how to hit it.

I mean, will you go out onto the court and remember to plug in these equations before you play every single time?

What we call "pronation" is the overanalysis of tennis stroke production; I think it's a very subtle thing that people all do naturally on their serve.

Click to expand...

For sure. I think people think 'if I talk about pronation a lot I look like a genius'. Honestly pronation is such a small part of the whole service motion... I have never thought about doing it when I serve.

Jeez, just because you guys don't understand it doesn't mean you have to be rude about it. Its not like he is forcing it down your throat. Its just something he probably thought was interesting and decided to share. Its no worse than yet another thread on the Federer fh grip or on which bh is better.

Jeez, just because you guys don't understand it doesn't mean you have to be rude about it. Its not like he is forcing it down your throat. Its just something he probably thought was interesting and decided to share. Its no worse than yet another thread on the Federer fh grip or on which bh is better.

Click to expand...

Just because people are commenting in the negative doesn't mean they don't understand it.

Analysis is interesting... sometimes... if it's correct. I can't believe analysis that has the forearm responsible for 40% of the serve.

I believe Brian Gordon's analysis of the serve also indicates that upper arm rotation (aka shoulder internal rotation) contributes about 40%. The analysis by toly is very impressive, and where I would tend to disagree is that pronation is "everything" in the serve. Those of us who have experimented with more shoulder/pectoral stretch, use of legs, etc, have experienced the enhancements these provide, although it is very diffcult to quantify the exact amount.

I believe Brian Gordon's analysis of the serve also indicates that upper arm rotation (aka shoulder internal rotation) contributes about 40%. The analysis by toly is very impressive, and where I would tend to disagree is that pronation is "everything" in the serve. Those of us who have experimented with more shoulder/pectoral stretch, use of legs, etc, have experienced the enhancements these provide, although it is very diffcult to quantify the exact amount.

Click to expand...

It doesn't contribute 40% to the serve power though. If there wasn't a rotation of the upper arm, you wouldn't be able to hit the ball in!! You'd be hitting the ball with the side of the frame.

You don't swing with the forearm. Yes it has to happen on a serve but it's just part of the throwing motion.

I believe Brian Gordon's analysis of the serve also indicates that upper arm rotation (aka shoulder internal rotation) contributes about 40%. The analysis by toly is very impressive, and where I would tend to disagree is that pronation is "everything" in the serve. Those of us who have experimented with more shoulder/pectoral stretch, use of legs, etc, have experienced the enhancements these provide, although it is very diffcult to quantify the exact amount.

Click to expand...

Please be careful saying "also", because in this thread 40% is being attributed to the forearm, not the shoulder, which is what you're talking about.

The analysis by Toly shows something simple: you get the most racket speed with the rotation of the entire arm and the racket head angled with the arm. Just think of a propeller: the shaft rotates slowly, but the outer edge of the propeller very fast.

It is the cumulative pronation of the upper arm, forearm, hand which sets the speed.

This effect can be accentuated having the proper grip for the serve, i.e., one that places the forearm and the racket in more of an angled position.